Reactive power is present in all electric power systems that transfer alternating current. Many loads consume not only active power but also reactive power. Transmission and distribution of electric power per se entails reactive losses as a result of series inductances in transformers, overhead lines and cables. Overhead lines and cables also produce reactive power as a result of capacitive connections between phases and between phases and earth potential.
At stationary operation of an alternating current system, active power production and consumption must be in agreement in order to obtain nominal frequency. An equally strong coupling exists between reactive power balance and voltages in the electric power network. If reactive power consumption and production are not balanced in a suitable manner, the consequence may be unacceptable voltage levels in parts of the electric power network. An excess of reactive power in one area leads to high voltages, whereas a deficiency leads to low voltages.
Contrary to active power balance at a nominal frequency, which is controlled solely with the aid of the active power control of the generator, a suitable reactive power balance is obtained with the aid of both controllable excitation of synchronous generators and of other components spread out in the system. Examples of such (phase compensation) components are shunt reactors, shunt capacitors, synchronous compensators and SVCs (Static Var. Compensators).
The location of these phase compensation components in the electric power network affects not only the voltage in various parts of the electric power network, but also the losses in the electric power network since the transfer of reactive power, like the transfer of active power, gives rise to losses and thus heating. It is consequently desirable to place phase compensation components so that losses are minimized and the voltage in all parts of the electric power network is acceptable.
The shunt reactor and shunt capacitor are usually permanently connected or connected via a mechanical breaker mechanism to the electric power network. In other words, the reactive power consumed/produced by these components is not continuously controllable. The reactive power produced/consumed by the synchronous compensator and the SVC, on the other hand, is continuously controllable. These two components are consequently used if there is a demand for high-performance voltage control.
The following is a brief description of the technology for phase compensation with the aid of synchronous compensator and SVC.
A synchronous compensator is in principle a synchronous motor running at no load, i.e. it takes active power from the electric power network equivalent to the machine losses.
The rotor shaft of a synchronous compensator is usually horizontal and the rotor generally has six or eight salient poles. The rotor is usually dimensioned thermally so that the synchronous compensator, in over-excited state, can produce approximately 100% of the apparent power the stator is thermally dimensioned for (rated output) in the form of reactive power. In under-excited state, when the synchronous compensator consumes reactive power, it consumes approximately 60% of the rated output (standard value, depending on how the machine is dimensioned). This gives a control area of approximately 160% of rated output over which the reactive power consumption/production can be continuously controlled. If the machine has salient poles with relatively little reactance in transverse direction, and is provided with excitation equipment enabling both positive and negative excitation, more reactive power can be consumed than the 60% of rated output stated above, without the machine exceeding the stability limit. Modern synchronous compensators are normally equipped with fast excitation systems, preferably a thyristor-controlled static exciter where the direct current is supplied to the rotor via slip rings. This solution enables both positive and negative supply as above.
The magnetic circuits in a synchronous compensator usually comprise a laminated core, e.g. of sheet steel with a welded construction. To provide ventilation and cooling the core is often divided into stacks with radial and/or axial ventilation ducts. For larger machines the laminations are punched out in segments which are attached to the frame of the machine, the laminated core being held together by pressure fingers and pressure rings. The winding of the magnetic circuit is disposed in slots in the core, the slots generally having a cross section in the shape of a rectangle or trapezium.
In multi-phase electric machines the windings are made as either single or double layer windings. With single layer windings there is only one coil side per slot, whereas with double layer windings there are two coil sides per slot. By coil side is meant one or more conductors combined vertically or horizontally and provided with a common coil insulation, i.e. an insulation designed to withstand the rated voltage of the machine to earth.
Double-layer windings are generally made as diamond windings whereas single layer windings in the present context can be made as diamond or flat windings. Only one (possibly two) coil width exists in diamond windings whereas flat windings are made as concentric windings, i.e. with widely varying coil width. By coil width is meant the distance in arc dimension between two coil sides pertaining to the same coil.
Normally all large machines are made with double-layer winding and coils of the same size. Each coil is placed with one side in one layer and the other side in the other layer. This means that all coils cross each other in the coil end. If there are more than two layers these crossings complicate the winding work and the coil end is less satisfactory.
It is considered that coils for rotating machines can be manufactured with good results up to a voltage range of 10-20 kV.
A synchronous compensator has considerable short-duration overload capacity. In situations when electro-mechanical oscillations occur in the power system the synchronous compensator can briefly supply reactive power up to twice the rated output. The synchronous compensator also has a more long-lasting overload capacity and is often able to supply 10 to 20% more than rated output for up to 30 minutes.
Synchronous compensators exist in sizes from a few MVA to hundreds of MVA. The losses for a synchronous compensator cooled by hydrogen gas amount to approximately 10 W/kvar, whereas the corresponding figure for air-cooled synchronous compensators is approximately 20 W/kvar.
Synchronous compensators were preferably installed in the receiving end of long radial transmission lines and in important nodes in masked electric power networks with long transmission lines, particularly in areas with little local generation. The synchronous compensator is also used to increase the short-circuit power in the vicinity of HVDC inverter stations.
The synchronous compensator is most often connected to points in the electric power network where the voltage is substantially higher than the synchronous compensator is designed for. This means that, besides the synchronous compensator, the synchronous compensator plant generally includes a step-up transformer, a busbar system between synchronous compensator and transformer, a generator breaker between synchronous compensator and transformer, and a line breaker between transformer and electric power network.
In recent years SVCs have to a great extent replaced synchronous compensators in new installations because of their advantages particularly with regard to cost, but also in certain applications because of technical advantages.
The SVC concept (Static Var. Compensator) is today the leading concept for reactive power compensation and, as well as in many cases replacing the synchronous compensator in the transmission network, it also has industrial applications in connection with electric arc furnaces. SVCs are static in the sense that, contrary to synchronous compensators, they have no movable or rotating main components.
SVC technology is based on rapid breakers built up of semi-conductors, thyristors. A thyristor can switch from nonconductor to conductor in a few millionths of a second. Capacitors and reactors can be connected or disconnected with negligible delay with the aid of thyristor bridges. By combining these two components reactive power can be stoplessly either supplied or extracted.
A SVC plant typically consists of both capacitor banks and reactors and since the thyristors generate harmonics, the plant also includes harmonic filters. Besides control equipment, a transformer is also required between the compensation equipment and the network in order to obtain optimal compensation from the size and cost point of view. SVC plant is available in size from a few MVA up to 650 MVA, with nominal voltages up to 765 kV.
Various SVC plant types exist, named after how the capacitors and reactors are combined. Two usual elements that may be Included are TSC or TCR. TSC is a thyristor-switched reactive power-producing capacitor and TCR is a thyristor-switched reactive power-consuming reactor. A usual type is a combination of these elements, TSC/TCR.
The magnitude of the losses depends much on which type of plant the SVC belongs to, e.g. a FC/TCR type (FC means that the capacitor is fixed) has considerably greater losses than a TSC/TCR. The losses for the latter type are approximately comparable with the losses for a synchronous compensator.
It should be evident from the above summary of the phase compensation technology that this can be divided into two principal concepts, namely synchronous compensation and SVC.
These concepts have different strengths and weaknesses. Compared with the synchronous compensator, the SVC has the main advantage of being cheaper. However, it also permits somewhat faster control which may be an advantage in certain applications.
The drawbacks of the SVC as compared with the synchronous compensator include:                it has no overload capacity. In operation at its capacitive limit the SVC becomes in principle a capacitor, i.e. if the voltage drops then the reactive power production drops with the square of the voltage. If the purpose of the phase compensation is to enable transfer of power over long distances the lack of overload capacity means that, in order to avoid stability problems, a higher rated output must be chosen if SVC plant is selected than if synchronous compensator plant is selected.        it requires filters if it includes a TCR.        it does not have a rotating mass with internal voltage source. This is an advantage with the synchronous compensator, particularly in the vicinity of HVDC transmission.        
In order to achieve a more competitive electricity market many countries have deregulated, or are in the process of deregulating, the electricity market. This usually involves a separation of power production and transmission services into separate entities. When these two parts of the system are in different hands, the previously existing link between the planning of generation plants and transmission lines is broken. A generation plant owner may announce the closing of a generation plant at timescales which are, for hardware investments, very short, presenting the operators and planners of transmission services with major changes in both load flow patterns and the location of controllable reactive production/consumption resources at short notice. Consequently, there is a strategic need for a phase compensation unit that can be relocated, within short lead time, to an arbitrary node in the transmission system.
In countries where the electricity market has not been deregulated there may also exist a need to have relocatable phase compensation components. For instance, countries with a large share of nuclear power production may encounter situations similar to that described above. Normally, nuclear power plants are closed down once a year during a low load season, for inspections and reparations. However, occasionally these plants may have to stay closed for longer periods of time due to major reparations. Although this situation is easier to handle in a country which has not deregulated the electricity market, the size of a typical nuclear plant may imply that the changes in load flow patterns and the absence of controllable reactive production/consumption resources puts the operators of the transmission system in situations which are difficult to handle while maintaining prescribed security standards. There exists a need for a relocatable phase compensation unit also in these situations.
There exist today a small number of relocatable SVC plants, see e.g. the article “Relocatable static var compensators help control unbundled power flows” in the Magazine “Modern Power Systems”, December 1996, pages 49-54. In addition to the differences between a static and a synchronous compensator described above, the relocatable static compensator involves a number of containers, which requires a fairly large area at the site and which needs to be electrically interconnected at the site. But most importantly the relocatable static compensator can only be connected to nodes in the transmission system where a step-down transformer already is available, providing a fairly low voltage. In other words, the relocatable static compensator cannot be directly connected to the transmission system voltage (typically 130 kV and up).
Due to the number of components required in a synchronous compensator plant and in particular the up to now necessary presence of a transformer, synchronous compensator plants for high-voltage networks up to now have been realized solely as stationary plants. In case of change in an existing power network regarding the need for phase compensation the plant might be superfluous at its location or might be required to be designed and dimensioned different, or a plant might be required somewhere else in the network. This of course is a serious drawback with such a stationary plant.